The invention relates to a diesel-electric drive system and to a method for stationary load testing of a diesel engine of the diesel-electric drive system.
A diesel-electric drive system of this generic type is known from DE 10 2007 003 172 A1 and is illustrated in more detail in the form of an equivalent circuit in FIG. 1. In this equivalent circuit, 2 denotes a diesel engine, 4 a generator, in particular a permanent-magnet synchronous generator, 6 a DC-link converter and 8 a rotating-field machine, in each case, in particular a polyphase asynchronous machine. The DC-link converter 6 has two self-commutated pulse-controlled converters 10 and 12 on the generator side, and two self-commutated pulse-controlled converters 14 and 16 on the load side. These self-commutated pulse-controlled converters 10, 12 and 14, 16 are electrically conductively connected to one another on the DC voltage side by means of a DC-link capacitor 18, in particular a DC-link circuit 20 having a DC-link capacitor bank. The generator 4 has polyphase winding systems 22 and 24, which are each linked by means of a circuit breaker 26 and 28 to connections R1, S1, T1 and R2, S2, T2 on the AC voltage side of the two generator-side self-commutated pulse-controlled converters 10 and 12. The corresponding connections R1 and R2, S1 and S2, and T1 and T2 on the AC voltage side are each electrically conductively connected to one another by means of a braking resistor 30. The method of operation of this diesel-electric drive system, in particular the braking mode, is described in detail in DE 10 2007 003 172 A1, and therefore does not need to be described at this point. FIG. 2 likewise illustrates in more detail an equivalent circuit of the diesel-electric drive system. This diesel-electric drive system differs from the diesel-electric drive system shown in FIG. 1 in that implementations are indicated for the generator-side and load-side self-commutated pulse-controlled converters 10, 12 and 14, 16. These self-commutated pulse-controlled converters 10, 12, 14 and 16 are implemented by means of double-converter bridge arm modules 32. FIG. 3 shows an equivalent circuit of this double-converter bridge arm module 32 in more detail. The two generator-side self-commutated pulse-controlled converters 10 and 12 are implemented with the aid of three double-converter bridge arm modules 32 while, in contrast, three double-converter bridge arm modules 32 are used for implementation of each load-side self-commutated pulse-controlled converter 14 or 16. Nine double-converter bridge arm modules 32 are therefore connected in order to provide the DC-link converter 6 for the diesel-electric drive system.
According to the equivalent circuit of the double-converter bridge arm module 32 shown in FIG. 3, this double-converter bridge arm module 32 has two bridge arm modules 34 which are connected electrically in parallel on the DC voltage side. Each bridge arm module 34 has two semiconductor switches 36 and 38 which can be turned off and are connected electrically in series, in particular Insulated Gate Bipolar Transistors (IGBTs), which are each provided with a corresponding freewheeling diode 40 or 42. A connection, point between two semiconductor switches 36 and 38, which can be turned off and are connected electrically in series, in each case forms a connection of R1 and R2, S1 and S2, and T1 and T2 on the AC voltage side. The connections 44 and 46 on the DC voltage side of each double-converter bridge arm module 32 are each electrically conductively connected to a potential in the DC-link circuit 20 in the DC-link converter 6.
In the case of diesel-electric traction drives, for example diesel locomotives or mining trucks, the generator 4 which is fitted to this diesel engine 2 is used to supply energy for the drive motors 8. The electrical voltage of the generator 4 is changed by the generator-side self-commutated pulse-controlled converters 10 and 12 to a predetermined DC-link voltage, from which the load-side self-commutated pulse-controlled converters 14 and 16 supply the drive motors 8. During electrical braking, the power flow in the DC-link converter 6 is precisely reversed. The energy is supplied into the DC-link circuit 20 of the converter 6 through the load-side self-commutated pulse-controlled converters 14 and 16. Since the diesel engine 2 cannot absorb braking energy, the braking energy must be converted to heat by means of the braking resistors 30. For continuous power distribution, a voltage which is pulse-width-modulated by the two self-commutated pulse-controlled converters 10 and 12 is passed to the braking resistors 30.
In order to make it possible to check the performance of the diesel engine 2, for example after repair, a so-called self-load test is carried out (stationary load test on an internal combustion engine). In the case of mining trucks and Eastern-European and North American diesel locomotives, this self-load test is already standard. In the case of diesel-electric drive systems which have a synchronous generator with an electrically produced field and a downstream diode rectifier, this self-load test is carried out when the vehicle is stationary via the braking resistors, which are fed from the DC-link circuit by means of a converter, in particular a chopper. Since the performance of the electrical brakes in these vehicles in general corresponds at least to the diesel engine power when driving, a stationary load test such as this cannot be carried out without an additional device.
A diesel-electric drive system of this generic type does not allow a self-load test to be carried out up to the maximum diesel engine power without more powerful semiconductor switches, since a self-load test:    a) on the one hand must feed the entire diesel engine power electrically into the DC-link circuit of the DC-link converter as a current fundamental, and    b) this power must be simultaneously transmitted in the opposite direction out of this DC-link circuit again, in the form of current harmonics, by means of the braking resistors.
This leads to overloading of the power electronics in the diesel-electric drive system when the current fundamental amplitude has the current harmonics superimposed on it at specific switching times.
In order nevertheless to allow a self-load test to be carried, out on this diesel-electric drive system of this generic type, it would be necessary to install an additional braking chopper in the DC-link circuit of the DC-link converter, which braking chopper would have to be designed for 50% of the total braking power. This additional braking chopper would not only cause additional costs but would also increase the weight of the traction converter. Furthermore, sufficient space to allow the additional converter to be accommodated would have to be provided on a diesel-electric locomotive or a mining truck.
The invention is now based on the object of improving the diesel-electric drive system of this generic type such that there is no need for an additional braking chopper in order to carry out a self-load test.